MicroElectroMechanical systems (MEMS) routinely use suspended micromechanical moveable electrode structures as electrostatically actuated mechanical members for both sensor and actuator based devices. Different methods exist for creating a support structure to suspend a moveable electrode structure. One method for suspending such a moveable electrode uses cantilevered members that are fixed to a substrate on one end and fixed to the movable electrode structure on the other end. In an alternate embodiment, the cantilever is made of, or coated with a conducting material and the cantilever itself serves as the moving electrode. The mechanical flexibility of the cantilever (e.g. bending) and/or motion at the fixed end(s) (e.g. hinge or flexible connection) allows for the motion of the suspended electrode. In some cases, the sensor or actuator device is based on motion of cantilever as such without an additional movable structure at the end of the cantilever. Such cantilevers are typically fixed-free or fixed-simply supported cantilevers.
A second method of suspending one or more moveable electrodes utilizes a plurality of cantilevers that support a moveable member which either serves as a moveable electrode or has mounted upon it moveable electrodes. A fixed electrode serves as an actuator to control movement of the moveable electrode structure through the application of an electric potential difference between the fixed electrode and the moveable electrode structure. The fixed electrode is typically positioned beneath the suspended moveable electrode to form a parallel plate capacitor like structure, with the fixed electrode acting as a first plate and the suspended moveable electrode acting as a second plate. The electric potential applied to the electrodes generates electrostatic forces that move or deform the support mechanism supporting the moveable electrode or the moveable electrode itself. Such support mechanisms may include bendable or otherwise deformable cantilevers.
Typical cantilever applications include micro sized relays, antennas, force sensors, pressure sensors, acceleration sensors and electrical probes. Recently, considerable attention has been focused on using cantilever arrays to develop low power, finely tunable micro-mirror arrays to redirect light in optical switching applications. Such a structure is described in U.S. Pat. No. 6,300,665 B1 entitled “Structure for an Optical Switch on a Silicon on Insulator Substrate” hereby incorporated by reference.
One problem with such cantilever structures is the limited amount of controllable motion that can be achieved with traditional arrangements of the cantilever and electrode. When a voltage difference is applied between two electrically conducting bodies separated by an insulating medium (for example air), the electrostatic force between the two bodies is inversely proportional to the square of the distance between the bodies. Thus when the moveable electrode is moved in closer proximity to the fixed electrode, as often occurs when a greater range of motion is attempted, strong electrostatic forces between the fixed electrode and the moveable electrode results in a “pull-in” or “snap-down” effect that causes the two electrodes to contact. The problem is particularly acute in D.C. (direct current) systems compared to A.C. (alternating current) systems.
In moving the electrodes, instability theoretically occurs in parallel plate capacitor structures when the movably suspended plate has traveled one third of the potential range of motion (typ. equal to the height of the air gap). In stressed metal systems, as described in the previously cited patent application, the cantilevers are typically ‘curled’—as opposed to more typical ‘straight’ cantilevers. However, such instability usually occurs when the actuation electrode is placed underneath the cantilever and the cantilever moves approximately beyond one-third of its potential range of motion.
Various solutions have been proposed to correct the potential for suspended electrodes and the corresponding supports structures to “snap-down”. These solutions include the following: using charge drives (see Seeger, et. al, “Dynamics and control of parallel-plate actuators beyond the electrostatic instability”, Proc. Transducers '99, Sendai), adding capacitive elements in series (Seeger, et. al, “Stabilization of Electrostatically Actuated Mechanical Devices”, Proc. Transducers '97, Chicago) or creating closed-loop feedback systems using capacitive, piezoresistive or optical detectors (Fujita “MEMS: Application to Optical Communication”, Proc. of SPIE, '01, San Francisco). These methods extend the stable range of motion to varying degrees. However all these methods complicate fabrication of the cantilever and actuator mechanism thereby increasing fabrication costs and reducing reliability. Thus an improved method of moving a cantilever through a wide range of motion while avoiding instabilities is needed.